1. Field of the Invention
The present invention relates generally to color cathode ray tubes for use with color display devices in color television sets and computer monitors or the like, and more particularly to those color cathode ray tubes with improvements in shape of a shadow mask assembly used therein
2. Description of the Related Art
As shown in FIG. 1, a color cathode ray tube is configured including a glass valve comprising a panel portion 1 and a neck portion 2 as well as a funnel portion 3, a phosphorus-coated surface or fluorescent film 4 as formed on the inner surface of the panel portion 1, a shadow mask assembly 5 disposed at a specified location opposing and adjacent to the phosphor film, an internal magnetic shield (not shown) as formed into a shape along the inner wall of the funnel portion, and an electron gun 12 for production of three electron beams 13 toward the phosphor film 4. There are co-provided and disposed outside of the neck portion 2 a magnet 9 for purity adjustment and a center beam static convergence adjustment magnet 10 plus a side beam static convergence adjustment magnet 11, wherein said electron beams 13 operate exhibiting deflection in the presence of a magnetic field that is created by a deflection yoke 8 as disposed outside of the glass valve to thereby penetrate the perforated molded mask 6 letting the specified phosphor film 4 emit light.
In the color cathode ray tube, whether the accuracy of positional alignment of the shadow mask assembly 5 and the phosphor film 4 is good or bad can directly affect whether resultant display images are good or not in quality. More specifically, display images are deteriorated when deformation or the like takes place due to mechanical distortion and/or thermal distortion at the perforated molded mask 6 and the support frame 7 constituting the shadow mask assembly 5.
FIGS. 2 to 4 show one example of the shadow mask assembly of conventional color cathode ray tube. In FIG. 2 the reference numeral 40 designates such shadow mask assembly, 41 denotes a perforated molded mask, 44 represents a support frame, 45 indicates a frame-shaped portion, 46 shows a spring, and 47 is a projection (referred to as the "dowel" hereinafter) as formed at the frame-shaped portion. The perforated molded mask 41 comprises a curved surface portion 42 which has multiple electron beam penetration holes (not shown), and a skirt portion 43 that down goes from four peripheries of the curved surface portion 4, wherein the support frame 44 is structured from a rectangular frame-shaped portion 45 and springs 46 attached to the outer wall of the frame-shaped portion 45 for permitting attachment of the support frame 44 to inside of the panel portion (not shown) as well as a plurality of dowels 47 attached to the inner wall of the frame-shaped portion 45 for causing the inner wall of the frame-shaped portion 45 and the skirt portion 43 to come into contact engagement with each other.
FIG. 3 is a diagram showing a cross-sectional view at a location indicated by A-A' in FIG. 2. FIG. 4 is a sectional diagram at a location represented by B-B' in FIG. 2. The support frame 44 is of a nearly rectangle shape with nearby portions of four vertexes of such rectangle being connected by curved planes, which may resemble in shape substantially the L-shaped cross-section with the thickness t as shown in FIG. 3 and FIG. 4.
The prior art shadow mask assembly 40 has the dowels 47 that are formed on the side wall of the frame-shaped portion 45 of the support frame 44 and extend toward inside of said substantial rectangle. Here, the height h of the dowels 47 as measured from inside of the sidewall of the frame-shaped portion 45 will be called the "embossing amount". As shown in FIG. 3 and FIG. 4, the support frame 44 is of the shape which has those embossing portions of length values 1.sub.1, 1.sub.2 in the inside direction thereof. While lengthening these embossing portion lengths 1.sub.1, 1.sub.2 may enhance the mechanical strength of the support frame 44, an excessive increase in length is not achievable due to limitations received from the cathode ray tubes effective screen size.
Since said prior art shadow mask assembly 40 is formed when press-machining the perforated molded mask 41 as was shown in FIG. 2 in such a manner that respective upper edges (peripheries of the curved surface section 42) of the skirt portion 43 are slightly curved outwardly at the center portions as compared to the corner sections thereof, the clearance with respect to a contact plane between the skit portion 43 and the frame-shaped portion 45 becomes larger at the corners of the skirt portion 43 thereby necessitating additional provision of the dowels 47 at respective corners of the frame-shaped portion 45 while at the same time suffering from a problem in that the resulting perforated area of the curved surface portion 42 with respect to the size of the support frame 44 gets narrower to the extent corresponding to an increase in clearance.
In said prior art shadow mask assembly 40 the contact surfaces between the skirt portion 43 and the frame-shaped portion 45 become only those layout portions of the dowels at the corners thus limiting the substantial contact regions between the skirt portion and the frame-shaped portion, which leads to a problem that the magnetic resistivity, which is important among magnetic shield characteristics of color cathode ray tubes, is significant disadvantageously at said corners of the perforated molded mask 41 thus letting any effective magnetic shield characteristics be hardly obtainable.
Another problem faced with said prior art shadow mask assembly 40 is that since the skirt portion 43 is designed so that respective upper edges thereof (peripheries of the curved surface portion 42) are formed to be slightly curved outwardly at the center portion as compared to corners, the workability remains inferior when inserting for assembly the perforated molded mask 41 into the support frame 44 while simultaneously associating with the risk of deformation of the perforated molded mask 41 during assembly processes.
An object of the present invention is to provide a color cathode ray tube with a shadow mask assembly offering increased robustness against deformation due to mechanical and/or thermal distortion, which structure is high in accuracy of positional alignment with an associative fluorescent or phosphor film and also excellent in magnetic shield characteristics.